1. Field of the Invention
The present invention relates to a projection optical system and an image display apparatus.
2. Description of the Related Art
Image display apparatuses of projector type are known in which still images such as characters, pictures, and the like and moving images two-dimensionally displayed on a small-sized light valve are enlarged and projected for image display through a projection optical system. Examples of light valves include transmissive liquid crystal panels, DLP (Digital Light Processor), and LCOS (Liquid Crystal on Silicon) superior in contrast characteristics.
Examples of image display apparatuses include front projection types in which an image on the light valve is enlarged and projected onto a projection surface such as a reflective screen disposed at a distance from the apparatus, and then a reflected light is viewed and rear projection types in which a transmissive screen is disposed inside the apparatus as the projection surface and the image on the light valve is enlarged and projected from a back of the screen, and then the image is viewed from a front of the apparatus.
Patent Document 1 discloses a combination of a wide-angle lens disposed on an object side with a curved mirror having negative power as a projection optical system for the rear projection type.
Patent Document 2 discloses a projection optical system in which an intermediate image is formed using a lens system telecentric on the object side and the intermediate image is formed on the screen as an object image using a reflecting optical system having positive power.
Patent Document 3 discloses a projection optical system in which an image is formed on the screen using the lens system telecentric on the object side and a Mangin mirror having negative power.
Patent Document 1: Japanese Laid-Open Patent Application No. 2004-133483
Patent Document 2: Japanese Laid-Open Patent Application No. 2004-258620
Patent Document 3: Japanese Laid-Open Patent Application No. 2005-292813
The projection optical system disclosed in Patent Document 1 has a short back focal distance and is not telecentric on the object side, so that it is not readily applied to a reflective light valve such as LCOS. In fact, in Patent Document 1, DMD (Digital Mirror Device) is assumed to be used for image display.
The projection optical system disclosed in Patent Document 2 is telecentric on the object side and application to the reflective light valve is disclosed. However, a catoptric system has positive power, so that when magnification is to be increased, an optical path length for image forming is likely to be increased. Thus, in an image display apparatus of rear projection type, depth or height of the apparatus is likely to be increased.
The projection optical system disclosed in Patent Document 3 is telecentric on the object side and application to the reflective light valve is disclosed. However, the Mangin mirror having negative power is used for the catoptric system and manufacturing of the Mangin mirror is difficult in comparison with manufacturing of a normal mirror and a cost thereof is increased.